It has long been known to acoustically log open wellbores to determine the velocities of compressional ("P") waves and shear ("S") waves travelling through rock formations located in the wellbore region. Logging devices have been used for this purpose which normally comprise a sound source (transmitter) and one or more receivers disposed at pre-selected distances from the sound source. For example, Kitsunezaki has suggested one such device for use in shear wave logging wherein the transmitter is located 3.2 meters from the first of 5 receivers, which are spaced 1 meter apart down an elastic rubber tube which is intended to acoustically isolate these elements from each other. See "A New Method for Shear Wave Logging", by Choro Kitsunezaki, Oyo Technical Note RP-4101, Oyo Corporation, Urawa Saitama 336 Japan (October, 1978).
By timing the travel of compressional waves, shear waves, and/or tube waves between the transmitter and each receiver, it is normally possible to determine the nature of surrounding rock formations. In logging loosely consolidated formations, however, it is often difficult to distinguish between compressional, shear, tube and secondary waves which may comprise portions of a wave train arriving at a given receiver. The use of remotely spaced, multiple receivers is thus intended to aid in distinguishing between arriving wave fronts and from noise in the system. Multiple receivers permit the recognition of similar wave patterns and wave fronts which are received at each successive receiver. Since travel time differentials increase with increasing distance from the transmitter source, wave fronts and patterns which are closely spaced at proximate receiver locations will separate by the time of their receipt at remote receiver locations.
Various signal timing and wave front analysis methods have also been suggested for distinguishing between wave fronts received at a given receiver. Most of these methods involve timing circuits which anticipate the receipt of, and facilitate the collection of, such wave front information. For descriptions of various logging techniques for collecting and analyzing compressional wave, shear wave, tube wave, and secondary wave data, please refer to U.S. Pat. No. 3,333,238 (Caldwell), 3,362,011 (Zemanek, Jr.), and U.S. Reissue No. 24,446 (Summers).
In the design of logging tools, various types of transmitters, such as, piezoelectric or magnetostrictive transmitters, have been suggested for creating acoustic logging signals. For conventional logging operations, most such transmitters have been centrally located in the borehole, and have been adapted to generate sound which is radiated in a multidirectional (360.degree.) pattern from the transmitter to adjacent wellbore surfaces. Such transmitters are well suited for creating compressional waves in surrounding rock and sand formations.
Since compressional waves travel faster than those shear, tube or secondary waves which may also be produced by a multidirectional transmitter, calculation of compressional wave velocity is accomplished by presuming that the first arriving wave front or wave pattern is that of a compressional wave. In loosely consolidated formations, subsequent arrivals of shear waves, tube waves and/or secondary waves are difficult to distinguish. In such formations, multidirectional transmitters tend to generate compressional waves of much greater amplitudes than any shear waves also produced thereby. Recognition of shear wave arrivals, is thus particularly difficult.
Recently, attention has been directed to developing transmitters which are particularly suited to shear wave logging. Such transmitters generally attempt to achieve a single point source application of sound energy to the borehole wall. The theory behind point source transmitters, as generally outlined in the above-mentioned Kitsunezaki paper, is that they are capable of directly generating S waves. Conventional multidirectional transmitters are said to be capable only of indirectly creating shear waves. Accordingly, point source type transmitters produce shear waves of substantially higher amplitudes than heretofore possible with conventional multidirectional P wave transmitters. Accordingly, formations, such as loosely consolidated or unconsolidated sand, which do not propagate shear waves in sufficient amplitudes to permit definitive detection using conventional P wave receivers, may now be shear wave logged with these S wave logging systems. Oyo Technical Notes RP-4105, entitled "Development of a Suspension Type S-Wave Log System," By Kimio Ogura (November 1979) and RP-4125, entitled "Development of the Suspension S-Wave Logging System (Report No. 2)", by Kimio Ogura, et al (November 1980) provide additional information relating to S-wave logging systems.
In spite of the above described developments in logging techniques and apparatus, difficulty is nonetheless encountered in logging open boreholes, particularly those boreholes disposed through unconsolidated or loosely consolidated formations, such as sandstone or sand. In order to obtain the best possible logging data, it is important to log boreholes as soon as drilling is completed. Drilling mud has a tendency to damage the logging characteristics of an open borehole, and may interfere with the gathering of reliable logging data if time is permitted to elapse between the completion of drilling and the logging operation. Not only is rig time extremely costly (currently on the order of $50,000 to $100,000 per day) but there are inherent dangers in prolonging any logging operation. For example, many boreholes will not stay open for extended periods of time. It is thus important to expedite the logging operation to obviate any necessity to reinsert pipe into the borehole to flush with mud. Accordingly, a need exists with loosely consolidated formations to obtain all data which may be collected through the use of both conventional and shear wave logging apparatus, and to do so in a manner which permits the efficient, reliable collection of such data from an open, recently drilled borehole.